Fabrication of photosensitive couplers

Abstract

Systems devices and methods in accordance with the invention impart high strength index of refraction patterns to photosensitive optical devices, such as Bragg gratings written in optical fibers. A length of small diameter fiber retaining photosensitivity is fabricated by flame elongation of an optical fiber precursor having dopant containing cladding, using a diffuse, low velocity inverted flame that does not introduce water, OH or H 2 into the fiber. By varying the flame velocity during each scan the fiber is diminished to a small, uniform diameter, waist region. Photosensitivity is preserved and enhanced by exposure of the prepared waist region to scanning actinic illumination within an in-diffusing environment of pressurizing hydrogen or deuterium, and controlling the exposure to optimize the photo-induced index change.

What is claimed is:  
     
         1 . The method of flame heating a length of photosensitive optical fiber during elongation of the fiber to reduce the fiber diameter while retaining the photosensitivity thereof, comprising the steps of: 
 mixing CO and O 2  gases;    directing the gas mixture as a low velocity flame toward the fiber as it is being elongated; and    moving the flame along the fiber to limit localized temperature buildup as the fiber is elongated.    
     
     
         2 . The method as set forth in  claim 1  above, wherein gas mixture is substantially OH free and includes an inert gas.  
     
     
         3 . The method as set forth in  claim 1  above, wherein the flame is directed downwardly toward the fiber and wherein the flame temperature at the fiber is about 2000° C.  
     
     
         4 . The method as set forth in  claim 1  above, wherein the step of moving the flame comprises reciprocating the flame along the fiber with velocity modulation of the flame movement such that flame residence time at reversal points along the reciprocation range is minimized.  
     
     
         5 . The method as set forth in  claim 4  above, wherein the steps of reciprocating the flame with velocity modulation comprise reciprocating with a given first velocity in the midregion of the span of reciprocation, rapidly decelerating and accelerating during direction reversals, and moving at a second velocity higher than the first velocity in the span between the midregion and the points of direction reversals, and wherein the span of the reciprocating movement is increased until a waist region of selected diameter is established.  
     
     
         6 . The method as set forth in  claim 1  above, further including the step of adjusting the flame temperature by adding an inert gas and varying the percentage of inert gas in the flame.  
     
     
         7 . The method as set forth in  claim 6  above, wherein the inert gas is nitrogen and the flame is distributed over an area with cross-sectional dimensions at least one order of magnitude greater than the width of the fiber.  
     
     
         8 . The method as set forth in  claim 7  above, wherein the flame is distributed by passing the gas mixture through an areally distributed permeable barrier, and wherein the gases are mixed in an approximately stoichiometric ratio.  
     
     
         9 . The method as set forth in  claim 8  above, wherein the O 2 /CO ratio is between 0.5 and 1, wherein the gas flow to the barrier is approximately from 50 to 150 scan, and wherein the CO is substantially free of reaction with iron and iron-containing compounds.  
     
     
         10 . The method of heating a glassy optical fiber during stretching to a smaller diameter comprising the steps of: 
 maintaining a stretching tension on the fiber;    directing an areally distributed flow of a burning gas mixture in downward direction toward the fiber; and    reciprocating the flow of burning gas along the length of the fiber with varying velocity and direction along increasing lengths of span until the fiber is stretched to a selected diameter waist region diverging in a taper at each end to the nominal fiber diameter.    
     
     
         11 . The method as set forth in  claim 10  above, wherein the burning gas comprises a substantially OH free mixture of combustible gas and O 2  at an initially approximately stoichiometric ratio, and the flow of the flame is at low velocity.  
     
     
         12 . The method as set forth in  claim 11  above, wherein the volume of the flame about the fiber has an interior temperature region of approximately 2000° C. and the flame is positioned with the fiber maintained in said interior temperature region.  
     
     
         13 . The method as set forth in  claim 12  above, wherein the burning gas includes CO and an inert gas and the method further includes the step of diffusing the low velocity flame over an area substantially greater than the width of the fiber, the volume of flame having expanding zones varying from stoichiometric to oxygen dominant concentrations, with the fiber being positioned in a maximum temperature zone that is close to stoichiometry in concentration.  
     
     
         14 . The method as set forth in  claim 13  above, wherein the O 2 /CO ratio at said interior temperature region is between 0.5 and 1, to sufficiently oxidize the fiber.  
     
     
         15 . The method of providing an optical fiber with an index of refraction pattern recorded therein comprising the steps of: 
 preparing adjacent core-cladding optical fibers, each with a photosensitizing dopant distributed through the cladding;    providing a combustible gas mixture with low potential OH content; establishing a distributed areal flow pattern of low velocity burning gas mixture directed toward the fibers;    maintaining the fibers under tension while reciprocating the flame along the fibers to establish localized states of plasticity in the fibers;    continuing the reciprocation and tensioning until the fibers elongate to form a central fused length of small cross-section bounded by diverging tapered sections;    diffusing a photosensitizing gas into the fibers at elevated temperature and pressure;    illuminating the central length of the fibers with a selected pattern of actinic light while continuing the heat and pressure to induce photorefractive and photo absorption effects; and    continuing the illumination at above a threshold level until the writing of a photorefractive index of refraction pattern is substantially complete.    
     
     
         16 . The method as set forth in  claim 15  above, wherein the step of illuminating comprises writing a grating with UV radiation in the central length of the fibers.  
     
     
         17 . The method as set forth in  claim 16  above, wherein the flame is reciprocated from an initial span of about 4 mm to a final span of about 24 mm, and the gas mixture is passed through a flow impedance diffusing filter before combustion.  
     
     
         18 . The method as set forth in  claim 17  above, wherein the flame is comprised of a mixture that does not substantially induce water, H 2  or OH into the fiber.  
     
     
         19 . The method as set forth in  claim 15  above, wherein the gas mixture comprises CO and O 2 , and wherein the flame scanning velocity relative to the length of the fiber is greater between turn around points and the center of reciprocation than at the center of reciprocation.  
     
     
         20 . The method as set forth in  claim 15  above, wherein the coupler elongation is performed in an ambient environment in which humidity is controlled to ±3% RH and temperature to ±2° C.  
     
     
         21 . A method of fabricating a fused coupler with a coupler waist of substantially constant cross-sectional dimensions, including the steps of elongating the coupler under a stable, flame substantially free of perturbations; reciprocating the flame in an increasing amplitude reciprocation pattern with velocity a minimum at the center of reciprocation and maximum at the endpoints of reciprocation, the increasing amplitude reciprocation pattern exhibiting a final amplitude of reciprocation greater than the desired length of the uniform coupler waist region.  
     
     
         22 . A method in accordance with  claim 21  above, including the steps of passing combustible gases through a diffusing filter immediately before entering the flame region to stabilize the flame characteristics over time.  
     
     
         23 . A method in accordance with  claim 22  above, wherein the final amplitude of reciprocation is about 24 mm and the uniform coupler waist region is of the order of 10 mm in length.  
     
     
         24 . A method in accordance with  claim 23  above, wherein the minimum velocity is about 1000 μm/s and the maximum velocity is about 20,000 μm/s.  
     
     
         25 . A method in accordance with  claim 24  above, wherein the uniform coupler waist region is uniform in cross-sectional dimension to within about 0.25 μm.  
     
     
         26 . A torch for use in heating an optical fiber to induce plasticity so that it can concurrently be stretched under tension, comprising: 
 a first chamber for receiving a combustible gas mix, the first chamber being spaced apart from the fiber;    a second chamber in interior communication with the first, and having an operative surface wall proximate the fiber to be stretched, the second chamber receiving the combustible gas mix from the first chamber, and    a gas permeable diffuser element disposed in the operative surface wall of the second chamber and providing a pressure attenuating, areal flow distributor outlet for the combustible gas mix, the flow from the second chamber through the diffuser element being directed at the fiber.    
     
     
         27 . A torch as set forth in  claim 26  above, wherein the second chamber is of ceramic material and the diffuser element is a porous element of heat resistant material.  
     
     
         28 . A torch as set forth in  claim 27  above, wherein the diffuser element is of material from the class comprising silicon carbide, alumina, platinum and the like and the second chamber is of material from the class comprising ceramics, alumina, metals and the like.  
     
     
         29 . A torch as set forth in  claim 26  above, wherein the second chamber is positioned to direct flow downwardly onto the fiber and wherein the diffuser element has pores of 30 to 100 microns in size to restrict flow to low velocity.  
     
     
         30 . A torch as set forth in  claim 29  above, wherein the diffuser element is 3 mm×6 mm in area, the second chamber is cylindrical and has outer dimensions of about 1.27 cm in diameter and about 1.27 cm in length, and includes an end cap with an aperture receiving the diffuser element.  
     
     
         31 . The method of writing a strong grating in an optical fiber comprising the steps of: 
 preparing a core/cladding fiber with a germanium doped, photosensitive cladding;    heating the fiber during stretching with a moving flame that imparts substantially no OH and oxidizes the fiber to a selected level;    stretching the fiber during heating until a waist region with a vestigial core is formed;    diffusing a photosensitizing gas into the waist region at elevated pressure;    maintaining the diffusion into the waist while writing a grating in the waist with scanning actinic illumination;    balancing the photochemistry by adjusting illumination intensity and scanning velocity to achieve a [Ge 2+ ] fraction of 0.1 or less; and    maintaining the illumination until the photo-induced index change reaches a selected level.    
     
     
         32 . The method of writing a grating set forth in  claim 31  above, wherein the scanning actinic illumination is laser UV illumination and wherein the photochemistry balance is optimized by varying UV laser intensity and exposure scan velocity to achieve a predetermined blue light luminescence variation as a function of the position of the UV illumination along the waist region.  
     
     
         33 . The method of writing a grating set forth in  claim 32  above, wherein the UV laser intensity is about 2 W/mm 2  at 244 nm and the laser scan velocity is about 0.5 mm/sec.  
     
     
         34 . The method of writing a grating as set forth in  claim 31  above, wherein the local duration of actinic illumination is varied in accordance with the degree of oxidation of the waist after stretching, and wherein the [Ge 2+ ] fraction is ascertained by measuring the value of the luminescence power.  
     
     
         35 . The method of writing a grating as set forth in  claim 40  above, including the added step.of enhancing the index change by maintaining the diffusion of photosensitizing gas for a time before illumination sufficient to increase the initial actinic absorption.  
     
     
         36 . The method of imprinting an apodized Bragg grating in a photosensitized waveguide with uniform average index of refraction and minimal chirp, comprising the steps of: 
 placing an apodized Bragg grating phase mask adjacent the fiber; directing a uv laser beam spot toward the coupler in scanning fashion through the phase mask and along the length of the coupler, wherein the exposure is varied during a lengthwise scan by:    ramping up the exposure to a start region of the photomask;    varying the exposure along the phase made to compensate for index of refraction variations introduced by the photomask and coupler non-linearity in response to exposure;    ramping down the exposure after the end region of the phase mask; and    repeating the scan along the coupler through the phase mask until the index of refraction pattern achieves a desired response.    
     
     
         37 . The method as set forth in  claim 36  above, further including the steps of sensing photoluminescence generated in the coupler by the laser beam, and varying the exposure along the phase mask to compensate for variations in the average index of refraction change during each scan and variations introduced by local non-uniformities during successive scans.  
     
     
         38 . The method as set forth in  claim 37  above, wherein the exposure is varied by varying the scan velocity of the beam.  
     
     
         39 . The method of imprinting an index of refraction grating in the waist region of a photosensitized waveguide with a uv laser beam comprising the steps of: 
 forming a uv laser beam having a transverse dimension sufficiently greater than the transverse dimension of the waist region to encompass the waist region of the coupler despite relative variations in the position of the beam relative to the waist region;    spatially filtering the beam to minimize sidelobes and reduce high frequency variations;    interposing a grating defining photomask in the path of the laser beam coextensive with the grating-receiving portion of the waist region, the grating defining variations within the photomask lying along the length of the waist region;    sensing photoluminescence from the waveguide that is generated by impingement of the laser beam on the waist region;    scanning the laser beam repeatedly with velocity modulation along the length of the waveguide wherein the velocity is varied to provide smoothly increasing exposure to the start of a principal scan length along the waist region through the photomask, and smoothly decreasing exposure after the principal scan length, and    varying the beam velocity along the principal scan length in accordance with the sensed luminescence to correct for non-linearities in photosensitivity and local non-uniformities.    
     
     
         40 . The method set forth in  claim 39  above, wherein the waveguide waist region is in the range of 5-10 μm in transverse dimension, the laser beam has a transverse dimension of approximately 60 μm and a lengthwise dimension of approximately 700 μm to approximately 1000 μm, wherein the photomask provides an apodized grating, wherein the step of sensing photoluminescence comprises sensing luminescence at approximately 500-700 nm, wherein the step of repeatedly scanning comprises scanning unidirectionally from one side with delay between scans, and wherein the method further comprises the steps of measuring the grating response and reiterating scanning if the response is inadequate relative to a selected standard.  
     
     
         41 . The method of writing an index of refraction grating through a grating pattern in a selected length of a photosensitive optical element comprising the steps of: 
 directing a beam of actinic radiation toward the optical element along the direction of the selected length to generate luminescence in the element;    varying the velocity of scan along the element to provide an entry exposure increasing to a selected level at the initial region of the selected length and an exit exposure decreasing to ineffectiveness after the selected length;    sensing the intensity of the luminescence generated in the element between the exit and entry;    varying the velocity of the scan along the selected length in accordance with the luminescence sensed, and    repeating the scan in the same direction with entry and exit velocity variations, and with velocity variations along the selected length to provide a substantially constant average index of refraction change across the grating.    
     
     
         42 . The method of scanning along a photomask with a laser beam which impinges on a photosensitive fiberoptic device to write a periodic index of refraction pattern determined by the photomask with minimum discontinuities recorded in the written pattern comprising the steps of: 
 establishing a laser beam pattern having a length in the direction of scan of a substantial fraction of the photomask length and having less than 1 mrad divergence;    directing the laser beam in a scanning motion with a prescan acceleration from a starting position to a first peak, and postscan deceleration from a second peak of like amplitude to the first;    blocking the laser beam from the photomask until the first peak is reached;    after the first peak, decelerating the beam to a variable control velocity;    sensing an exposure parameter of the fiber optic device at the control velocity;    varying the scan for a predetermined length along the photomask in accordance with the control parameter to correct factors that introduce discontinuities;    after the photomask, accelerating the beam to the second peak;    blocking the laser beam at the second peak;    decelerating the laser beam to the starting position, and    continuing the scan sequence until the desired index of refraction pattern is written.    
     
     
         43 . The method as set forth in  claim 42  above, wherein the fiber optic device has a transverse dimension of about 5-10 μm, wherein the laser beam pattern has a transverse dimension of about 60 μm and a longitudinal dimension of about 700 μm -1000 μm, wherein the prescan acceleration, post scan acceleration, deceleration after the first peak and acceleration until the second peak are all substantially linear, wherein the beam is of substantially constant intensity, and wherein the step of varying the scan comprises compensating for variations in the average index of refraction change and concurrently variations introduced by local non-uniformities.